In disc drive data storage systems, data is stored along tracks on a disc. In many systems, data is stored in blocks on the tracks to allow for localized error detection and correction during reading. Usually, the data blocks are grouped into sectors that are marked by a timing reference mark. In order to access a particular data block in a sector, such systems initiate a timer when the timing reference mark is detected. When the timer expires, the read/write head of the disc drive data storage system is assumed to be over the desired data block.
A problem that is encountered when using this indirect addressing scheme to locate a data block is that variations in the rotational speed of the disc and variations in the processing time required to identify the timing reference mark can cause an incorrect correlation between the timer and the actual position of the read/write head. As a result, the timer may expire before the read/write head reaches the data block or after the read/write head has already passed the beginning of the data block.
In order to prevent one data section from being written onto another data section as a result of an incorrect correlation between the timer and the actual position of a write head, it is known to insert empty buffer fields, known as spin pads, between the data blocks on a track. In most disc drive data storage systems, all the spin pads along a track have the same length. The required length of a spin pad to ensure that one data section is not written onto another data section, however, is a function of the distance of the spin pad from the timing reference mark, such that the greater the distance of a spin pad from the timing reference mark, the greater the required length of the spin pad. As a result, in disc drive data storage systems that use a fixed pad length, the spin pads between all data blocks must have a length that is as long as the longest spin pad length that is required. The use of unnecessarily long spin pads between most data blocks takes up space that could otherwise be used to store data, and is an obstacle to increasing data capacity in a disc drive data storage system.
In commonly assigned, copending application entitled “METHOD AND APPARATUS PROVIDING VARIABLE SPIN-PAD LENGTHS”, Ser. No. 09/977,098, filed Oct. 12, 2001, a disc drive data storage system is disclosed that includes a track layout having spin pads of different length between different data blocks. The length of each spin pad on a track is a function of the distance of the spin pad from the timing reference mark, and the length of each spin pad increases as the distance of the spin pad from the timing reference mark increases.
By varying the length of the spin pads as a function of distance from the timing reference mark, the total space occupied by the spin pads on a track is reduced, thus providing additional space on the track to store data.
In order to maximize the space available on a track to store data, the length of each spin pad on a track should be only as long as is required to prevent alignment errors. The above-mentioned commonly assigned copending application describes methods for calculating the proper lengths of spin pads to be written on a track as a function of distance of the spin pads from a timing reference mark. It would be desirable, however, to provide a disc drive data storage system that includes a mechanism for determining the length of a spin pad to be written on a track as a function of distance from a timing reference mark during an operation of writing a data section on the track. The present invention provides a solution to this and other problems, and offers other advantages over previous solutions.